Prior art terminated bus systems typically have one or more bus cycles (e.g., dead cycles) during which bus agents transfer control of the bus. During the dead cycle no bus agent drives the bus. Because the bus is terminated, the terminating components pull the bus lines up to a predetermined voltage level. The length of the dead cycle is determined, in part, on how quickly the terminating components can settle transient signals on the bus. In general, settling refers to the lines of the bus reaching a predetermined voltage level and overcoming ringbacks, reflections and other transients.
Transients may occur as the result of switching control of the bus lines from a first bus agent to the terminating components and again when switching control from the terminating components to a second bus agent. When the first bus agent gives up control of the bus, the first bus agent tri-states the input/output (I/O) buffers coupled to the bus by placing the buffers in a high-impedance state.
The terminating components take control of the bus lines as left by the first bus agent when the first bus agent's bus access cycle is completed. The terminating components then pull the bus lines up to a known voltage. The terminating components drive the bus lines to the known voltage at least until any transients on the bus have settled. Because during normal bus operation the termination components provide termination and the bus agent provides source termination, settling of transients takes longer for dead cycles than for normal bus cycles. In other words, during normal bus operation, more termination components are operating than during dead cycles, which results is faster settling of transients.
When a second bus agent takes control of the bus from the terminating components the second bus agent must overcome transients associated with the change of bus control before signals on the bus can be used to convey valid data. Because the second bus agent provides source termination in addition to the end termination provided by the termination components, the second bus agent settles the bus faster than the termination components alone during the dead cycle.
Because the time required to settle transient signals cannot be used to convey valid data, what is needed is a method and apparatus for decreasing the time during which the bus is not used to transfer valid data.